1. Field of the Invention
The present invention relates to an apparatus for dispensing electrostatically charged particles onto a substrate where they subsequently form a continuous and uniform coating. In one aspect the invention relates to an apparatus useful for coating substrates with the resinous powders having the improved electrostatic charge. In another aspect the invention relates to an apparatus which utilizes a induction/conduction charging system for coating substrates.
2. Background Art
In recent years, much progress has taken place in the field of electrostatic powder coating. Powder coating, as a separate technology, developed as a result of a number of clear advantages over other methods of coating such as brushing, dipping and conventional spraying. These include the inherent advantages due to the absence of solvent (safer, less harmful to the environment, less expensive, cleaner working environment) as well as decreasing the time taken for the coating process to produce an article ready for use. Control of the coating thickness and the ability to produce a high quality finish from a single application treatment are also possible with this method.
Much of the early work in the field resulted in methods being developed which are capable of reaping many of these advantages. However, there are still a number of drawbacks within the technology which need to be overcome.
Powder coating technology is based on the principle of electrostatic charging and presently available practical methods of charging are classified into a corona charging system, a tribo-electric charging system or a hybrid system. Each system has evolved from the earliest corona charging system which is little more than a hollow barrel through which powder is pneumatically conveyed, with charging of the powder being accomplished by ionic attachment at the barrel, or gun exit.
A brief review of each of the current systems and the reason for the development of the more recent tribo and hybrid systems is given here to serve as a background to the present invention.
The basic corona charging system involves charging by ionic bombardment using an ion source such as a high voltage corona electrode or radioactive element. This method is used quite often to apply charge to highly insulating materials such as plastics. It can be very inefficient when applying electrostatic charge to powders since many of the ions produced do not contribute to the charging of particles but alight elsewhere, for example, on the workpiece itself in a powder coating operation. In some of the worst cases, charging efficiencies of less than 1% had been quoted in corona powder coating equipment.
In the corona charging system, powder is conveyed from a hopper through feed hoses to a spray gun. A sharp pointed electrode in the gun is connected to a high voltage generator and the combination of electrode geometry and high voltage (up to 100 kV in some guns) creates an electric field in excess of the local breakdown strength of the surrounding gas, which is usually air. A corona discharge is generated and free ions are formed in front of the charging electrode. Powder particles are conveyed through this space charge region and are charged by ionic attachment. The particles follow the air-flow pattern and those that are sufficiently charged are deposited onto the workpiece, which is generally held at ground potential. The polarity of the charging electrode can be reversed to create either a positive or negative charge on the particle, with a negative charge being generally preferred due to the larger numbers of ions being produced.
The charging efficiency of this system is very poor since only a small fraction (-0.5%) of the ions produced by the corona contributes to the charge on the powder. The majority of the ions produced by the corona gun do not attach to the sprayed powder particles but travel as `free ions` to the workpiece where they accumulate rapidly within the deposited powder layer.
As more free ions reach the workpiece, the intensity of the charge within the powder layer reaches saturation. At this point small electrostatic discharges (back-ionization) can occur resulting in disruptions in the coating and, ultimately, a poor quality finish.
The onset of back-ionization essentially limits the useful coating thickness that can be applied using corona charging powder coating equipment.
Besides requiring a high voltage power supply, a further disadvantage of corona guns is that they are not suited for applications requiring penetration into cavities and corners. This is due to all the voltage which appears at the external high voltage electrode being dropped between the gun head and the grounded workpiece with subsequent little, or no, penetration of the field associated with this voltage into cavities and recesses. These areas then approximate enclosed Faraday cages. Under these conditions internal coating will only be achieved by pneumatically conveying the particles into such areas, which can be difficult to achieve while simultaneously ensuring good coating uniformity elsewhere.
Perhaps the most common alternative system to corona charging is triboelectrification or frictional charging which takes place when two unlike materials or surfaces which are previously uncharged, that is in a electrically neutral state, make contact and then separate. During this process electrostatic charge is also separated with one of the surfaces attaining a postive polarity charge and the other a negative charge. This process occurs commonly in everday life. Examples are powder being conveyed through a pipe and a person walking across a carpeted room. In the latter case, there is friction between the soles of the shoes and the carpet.
The magnitude and even the polarity of electrostatic charge generated in this way are heavily dependent on factors such as surface contamination, moisture content and the nature of the contact. Although this method of charge generation is used in electrostatic powder coating, it has encountered reliability problems.
While a standard corona gun applies a charge of approximately 1.times.10.sup.-3 C/kg to powder particles, frictional charging transfers a few hundreds of electronic charge per contact and, therefore, to obtain charges equal to a corona gun thousands of contacts are required. The simplest method by which this is achieved is a straight tube in which there is turbulent flow, resulting in a large number of powder/wall collisions. Wall surfaces are ideally insulators arranged with grounding points so the high charge built up on the surface can decay to ground. PTFE, poly(tetrafluroethylene), is usually used in commercial systems and its place in the tribo-electric series ensures that most powders charge to a positive polarity on contact with it.
With tribo-electric guns the free ion current is eliminated or considerably reduced and, as there is no applied electric field, the particles are directed onto the workpiece by a combination of the air flow and the field produced by the charged powder cloud. Due to these factors, back ionization does not occur for 10 to 20 seconds in tribo-electric systems and it is easier to obtain heavy or thick films with this system. A further advantage is the ability of the system to coat inside cavities, small complex parts and products with sharp corners, etc. Furthermore, frictional charging not only overcomes the Faraday cage effect and reduces back ionization, but facilitates gun design to accommodate spray heads that accept different types of nozzles.
The fundamental disadvantage with a tribo gun is that a decrease in efficient charge exchange occurs after a prolonged period of operation. A still further disadvantage is that the particle size distribution of the powder has a significant effect on tribo charging and its efficiency. A typical powder for coating contains a combination of small, medium and large particles, ranging from sub-micron size up to greater than 80 microns in diameter. It is known that within such systems hi-polar charging of the powder can occur, with smaller particles more likely to charge to a negative polarity. The efficiency of charging is a function of the diameter of the particle and as a result the smallest particles are not electrostatically attracted to the workpiece resulting in preferential deposition of the mid-size range particles. Thus transfer efficiency is reduced and so too the overall operational efficiency of the system due to the increasing build-up of deposits in the guns and powder collecting and recycling equipment. Fluidizing problems in the feed hopper can also occur.
Finally, there are the so called "hybrid" guns which contain both of the aforementioned methods i.e., corona charging and tribeelectrification in one gun, in an attempt to combine the advantages of both systems. However, this approach does not remove the main inherent disadvantages of both guns--poor powder charging and transfer efficiency.
The coating efficiency is about 70-75% at best using presently available materials for practical industrial purposes. Any non-deposited powder will be wasted or must be recovered by use of special recovery equipment and reused by adding it in small portions to virgin powder or by recycling it to the resin preparation step. Manufacturers of powder coatings claim that it is possible to achieve 97-98% usage of powders, citing this as an incentive for switching from wet spray systems where any overspray is wasted. A flaw in this argument is that to achieve such high usage dedicated recycle equipment must be operated on an exclusive basis on each line, whereby it is not easy to change the type or hue of the coating material. Thus, the installation cost of the recovery apparatus and the awkward scheduling of its operation and the time required for the recovery add to the total cost.
Accordingly, one or more of the following objects can be achieved by the practice of the present invention. It is an object of the present invention to provide an apparatus for electrostatically charging a powder for use in powder coating applications which is free from the aforementioned shortcomings. A further object of the invention is provide an apparatus for charging powders which allows an electrostatic charge to be developed on the powder in a reliable and repeatable manner. Another object is to provide an apparatus which can accurately and reliably control the quantity and polarity of electrostatic charge developed and thus insure the coating of all areas of a workpiece to any required thickness. Another object of the invention is to provide an apparatus for applying a charge to thermoplastic and thermosetting resins which are used in powder coating operations. Another object is to provide an apparatus wherein the electrostatic charge on powders is improved by incorporating an electrostatic property modifying agent in, or on, the surface of the resin. A still further object is to provide an apparatus for applying electrostatically charged powders as a coating on solid objects. A still further object is to an apparatus for coating solid objects by inductive means. Another object is to provide an apparatus for coating solid objects with a powdered resin which can be subsequently fused to provide a uniform and continuous coating on such objects. Another object of the present invention is to provide an apparatus for the application of powder coating to solid objects which is efficient and minimizes powder waste.
A further object of the invention is to provide a system useful for spraying the electrostatically charged powders onto solid objects which can then be fused to provide a permanent finish. Another object is to provide a novel system for spraying electrostatically charged powders onto heated solid objects whereby fusing of the powder into a permanent finish is achieved. These and other objects will readily be achieved in light of the teachings herein set forth.